Bump GitHub CI macos-12 to macos-13, due to upstream deprecation.

  - See [GitHub's deprecation plan](actions/runner-images#10721) which is entirely dropping support in approximately one week.
  - Bump up to the next supported version.

PiperOrigin-RevId: 700398760

.github/workflows/ci.yml

@@ -22,7 +22,7 @@ jobs:
         with:
           platform: macos
   macos-amd64:
-    runs-on: macos-12
+    runs-on: macos-13
     steps:
       - name: Checkout repo
         uses: actions/checkout@v4